Automobiles generally include an internal combustion engine that generates drive torque via combustion of an air and fuel mixture. The drive torque is transferred to a driveline through a transmission. More specifically, the transmission includes an output shaft that is coupled to driveline components including, but not limited to, a differential, driveshafts, propeller shafts and a transfer case. In hybrid electric vehicles, an electric machine is included to selectively produce drive torque to drive the vehicle or to assist the engine in driving the vehicle.
Clunk is an objectionable driveline noise and vibration phenomena that is induced by torque reversals. Clunk initiates drive train ringing and body and chassis low frequency vibration. As the driveline goes from a driven condition (i.e., negative torque) to a drive condition (i.e., positive torque), the lash in the coupled components (e.g., the transmission, transfer case, universal joints, constant velocity joints, propeller shaft, differential and axle shafts) has to be eliminated before positive torque can be transmitted. Lash is defined as the movement that occurs without resistance, which results from imperfections and tolerance in coupled components. The lash elimination process causes unrestricted and minimally damped rotation of the driveline components until resistance occurs. Following the lash removal, the driveline components go into an under-damped, spring-mass oscillation, which generates both noise and vibration through the vehicle body.
Clunk normally occurs during the torque reversal generated by application of the throttle following a deceleration fuel cut-off or a high engine vacuum fuel deceleration. In the case of a hybrid vehicle that shuts off fuel at low speeds and idle, clunk can result from the electric machine cranking the engine for restart. Clunk is most prevalent when the restart occurs while the vehicle is moving, but can occur even with stationary starts.
Traditional methods of controlling clunk severity seek to minimize the angular acceleration of driveline components during the period in which lash is being taken up. However, identifying the condition during which the driveline lash is being taken up is critical. This condition changes with vehicle speed, engine speed, torque converter characteristics and the amount of lash. Traditional clunk control systems are unable to accurately identify when this condition occurs and to effectively minimize driveline clunk.